Carrier multiplication

A key issue is the improvement of solar light harvesting. New active materials with high optical absorption in the visible and good photostability are needed. Implementation of carrier multiplication through impact ionization in quantum dots arrays could mitigate the losses related to carrier thermalization. The alternative approach is the development of vertically stacked tandem systems of increasing band gap active materials, which effect H+ reduction and water oxidation on opposite sides. 

Hanna MC, Nozik AJ (2006) Solar conversion efficiency of photovoltaic and photoelectrolysis cells with carrier multiplication absorbers. J Appl Phys 100 074510 (8 pages)... 

R. Schaller, V. Klimov, High efficiency carrier multiplication in PbSe nanocrystals Implications for solar energy conversion, Phys. Rev. Lett. 92 (2004) 186601. 

V.L. Rupasov, V.I. Klimov, Carrier multiplication in semiconductor nanocrystals via intraband optical transitions involving virtual biexdton states, Phys. Rev. B 76 (2007) 125321. 

MEG PV device is 44.4% for bandgap Eg = 0.7 eV, where = 6. This is similar to the value reported by de Vos and Desoete (1998) for a blackbody-illuminated PV device with maximum carrier multiplication. It is interesting to note that a high efficiency of -42% is still obtained with a multiplication of only 2 (curve labelled M2), which is 94% of the absolute maximum. This means that high multiplication values (>2) are not essential for substantially increasing the efficiencies of single-gap PV devices having carrier multiplication absorbers. 

Brendel R., Werner J. H. and Queisser H. J. (1996), Thermodynamic efficiency limits for semicondnctor solar cells with carrier multiplication , Sol. Energy Mat. Sol. Cells 41-2, 419-425. 

Franceschetti A., An J. M. and Zunger A. (2006), Impact ionization can explain carrier multiplication in PbSe quantum dots , Nano Lett. 6, 2191-2195. 

Schaller R. D., Agranovich V. M. and Klimov V. I. (2005a), High-efficiency carrier multiplication through direct photogeneration of multiexcitons via virtual single-exciton states , Nat. Phys. 1, 189-194. 

Schaller R. D., Petruska M. A. and KUmov V. 1. (2005b), Effect of electronic structure on carrier multiplication efficiency comparative study of PbSe and CdSe nanocrystals , Appl. Phys. Lett. 87,253102-253104. 

Spirkl W. and Ries H. (1995), Lnminescence and efficiency of an ideal photovoltaic cell with charge carrier multiplication , Phys. Rev. B 52, 11319-11325. 

Exciton energy transfer from organics to semiconductor nanocrystals and carrier multiplication... 

NCs contain approximately 100-10,000 atoms. Because of the strong spatial confinement of electronic wavefunctions and reduced electronic screening, the effects of carrier-carrier Coulomb interactions are greatly enhanced in NCs compared with those in bulk materials. These interactions open a highly efficient decay channel via Auger recombination and just such a strong carrier-carrier interaction in NCs is responsible for carrier multiplication (61)-(63). 

CARRIER MULTIPLICATION IN QUANTUM DOTS ACCOMPANIED BY RAMAN SCATTERING PHENOMENA... 

The carrier multiplication (CM) process generated as a result of a single photon absorption in a spherical quantum dot (QD) is explained as due to multiple,virtual band-to-band electron-photon quantum transitions. Only the electron-photon interaction is used as a perturbation without the participation of the Coulomb electron-electron interaction. The creation of an odd number of electron-hole (e-h) pairs in our model is characterized by the Lorentzian-type peaks, whereas the creation of an even number of e-h pairs is accompanied by the creation of one real photon in the frame of combinational Raman scattering process. Its absorption band is smooth and forms an absorption background without peak structure. It can explain the existence of a threshold on the frequency dependence of the carrier multiplication efficiency in the region corresponding to the creation of two e-h pairs. 

In series of papers [1-5] the process of carrier multiplication (CM) generated by the absorption of a single photon in spherical quantum dots of CdSe, PbS and PbSe semiconductors the condition of a strong size quantization was revealed. The energy spectrum and the optical band-to-band quantum transitions in quantum dots with spherical symmetry were studied taking into account the different band structure of the semiconductors [6-10]. 

Side by side with it, one can discuss also another mechanism of carrier multiplication (CM) without participation of the Coulomb interaction. It is based on the successive, multiple application of the electron-photon interaction... 

The proposed model of the carrier multiplication process in quantum dots permits to explain a possibility to create with considerable probability three electron-hole pairs. It shows the way to the two steps with creation of four and five e-h pairs. It can explain the existence of the threshold on the frequency dependence of the CM quantum efficiency, when two pairs and a Raman scattered photon are created. 

Trinh MT, Houtepen AJ, Schins JM, Hanrath T, Pins J, Knulst W, Goossens APLM, Siebbeles LDA (2008) In spite of recent doubts carrier multiplication does occur in PbSe nanocrystals. Nano Lett 8 1713-1718... 

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